Represents the "StateModel" entry of the text annotation object.

It is defined as part of the PDF 1.5 specification.

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 172, page 394.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 175, page 479.

The state model identifies which vocabulary of state values is in use. PDF defines two models: Marked (simple marked or unmarked) and Review (full review workflow with accepted, rejected, and other states).